The present invention relates to a wire or felt forming section of a paper making machine having rollers that guide the wire or felt through the initial part of the forming section. The wire or felt forming section includes an endless loop wire screen, usually called a wire, or an endless loop fabric layer, usually called a felt, that conveys and dewaters paper pulp suspension that is ejected from the headbox of a paper-making machine onto the wire or felt. Where the suspension is ejected into a gap between two wires or felts, that is a twin wire forming section. Hereafter, only a wire forming section is specifically discussed. But, a felt forming section is to be understood as included as well.
A wire or felt forming section of this type is known from German Patent Publication 3 815 470 A1.
When a wire in a paper making machine is very wide, i.e. wider than 9 m. for example, the rollers which direct the wire past the outlet slot or nozzle from the headbox, and which are known as breast rollers, must be 1 m. or more in diameter to reduce buckling of the wire. Two wires approach the headbox outlet slot, one from above and one from below, respectively. The wires are each wrapped around part of the circumference of a respective breast roller above and below the outlet slot. There are two parallel breast rollers disposed at opposite sides of the outlet slot for guiding the two wires. A gap is formed between the two wires just past the outlet slot by the placement of the breast rollers. A jet of paper pulp suspension is ejected from the headbox outlet slot into the gap between the two wires. Because of the large diameter of the rollers, the distance the paper pulp jet has to travel between the two wires from the headbox nozzle to the pulp gap, which is near where the circumferences of the cooperating breast rollers are closest together, is accordingly very long. In order to reduce this distance, a comparable system, which is disclosed in European Patent Publication 0 335 821 A2, has rigid lead through guides for the wires in the form of respective ceramic strips, instead of breast rollers. The wires rub against the strips considerably and will wear out rapidly.
A guiding, lead through or breast roller is generally a cylinder with a journal at each end. Because of the maximum permissible sag or to avoid critical or even semi-critical speeds, the diameter of such a cylinder is selected dependent upon the width of the machine and upon how rapidly the cylinder rotates. Lead through rollers with diameters of 0.7 to 0.9 m and breast rollers with diameters of 1000 to 1250 mm are now in common use. Such diameters are often much greater than what they should be for optimal design.
It is important to keep the pulp jet leaving the outlet slot of the headbox as free as possible of turbulence, of friction with the air, and of pulp flow components that are at an angle to the direction of pulp and wire travel. This requires keeping the distance the jet travels between the headbox outlet slot and the point at which it strikes the wire or wires of the forming section as short as possible. This point is at the beginning of the pulp gap in a twin wire machine. The breast rollers must accordingly be as small in diameter as possible.